Modern semiconductor technology is highly dependent on an accurate control of the various processes used during the production of integrated circuits. Accordingly, the wafers have to be inspected repeatedly in order to localize problems as early as possible. Furthermore, a mask or reticle should also be inspected before its actual use during wafer processing in order to make sure that the mask accurately defines the desired pattern. This is done because any defects in the mask pattern will be transferred to the substrate (e.g., wafer) during its use in microlithography. However, the inspection of wafers or masks for defects requires the examination of the whole wafer or mask area. Especially, the inspection of wafers during their fabrication requires the examination of the whole wafer area in such a short time that production throughput is not limited by the inspection process.
Scanning electron microscopes (SEM) have been used to inspect wafers to detect pattern defects. Thereby, the surface of the wafer is scanned using a single finely drawn electron beam. When the electron beam hits the wafer, secondary electrons are generated and measured. A pattern defect at a location on the wafer is detected by comparing an intensity signal of the secondary electrons to, for example, a reference signal corresponding to the same location on the pattern. However, because only one very narrow electron beam is used for scanning, a long time is required to scan the entire surface of the wafer. Accordingly, it is not feasible to use a conventional (single-beam) Scanning Electron Microscope (SEM) for wafer inspection, since this approach does not provide the required throughput. Therefore, high speed wafer inspection is presently carried out by means of light optical techniques.
In order to perform this task using electron microscopic techniques several approaches have been suggested. One approach is based on the miniaturization of SEMs, so that several miniaturized SEMs (in the order of ten to one hundred) are arranged in an array and each miniaturized SEM examines a small portion of the complete sample surface. Another approach makes use of fixed-beam surface electron microscopes which image a certain area of the sample simultaneously. These surface microscopes can be classified by the excitation process of the electrons that form the image at the detector: a) The Photoemission Electron Microscope (PEEM), where the electrons are created by illumination of the sample surface with UV light, synchrotron radiation, or X-rays and b) the so-called Low-energy Electron Microscope (LEEM), where, in various modes of operation, the sample surface is illuminated with electrons. In this case, the illuminating electrons have to be separated from the imaging electrons by means of an additional electron optical element, for example, a beam separator in the form of a dipole magnet. However, both approaches have not yet been put into industrial practice.
Multi-beam electron projection systems are used to create patterns of variable shape on a substrate by switching on and off individual beams as is described in document EP 0 508 151. The following remarks are particularly relevant: First, as a projection system, it inherently does not produce an image of a sample and, therefore, does not comprise an objective lens. Secondly, in the example to which we referred to above, a resulting electron beam, formed by the individual beams that are not blanked out, is scanned as a whole over the substrate.
Furthermore, SEMs using multiple charged particle beams have been suggested in order to increase the throughput of data collection process. For example, U.S. Pat. No. 5,892,224 describes an apparatus for inspecting masks and wafers used in microlithography. The apparatus according to U.S. Pat. No. 5,892,224 is adapted to irradiate multiple charged particle beams simultaneously on respective measurement points on the surface of a sample. However, the apparatus according to U.S. Pat. No. 5,892,224 is primarily designed for the inspection of masks and does not provide the resolution which is required to inspect the intricate features present on a semiconductor wafer.
In charged particle beam devices, such as a scanning electron microscope (SEM), the charged particle beam exhibits a typical aperture angle as well as a typical angle of incidence in the order of several millirads. However, for many applications, it is desirable that the charged particle beam hits the sample surface under a much larger angle of typically 5° to 10°, corresponding to 90 to 180 millirads. Stereoscopic visualization is an example of such an application. Some applications even require tilt angles in excess of 15° or even 20°. In many cases, the additional information which is contained in stereo images is extremely valuable in order to control the quality of a production process.
Thereby, a number of tilting mechanism can be used. In early solutions, an oblique angle of incidence was achieved by mechanically tilting the specimen. However, apart from other drawbacks, mechanically tilting the specimen takes a lot of time. An oblique angle of incidence may also be achieved by electrically tilting the charged particle beam. This can be done by deflecting the beam so that either by the deflection alone or in combination with the focussing of the beam, an oblique angle of incidence results. Thereby, the specimen can remain horizontal which is a significant advantage as far as the lateral coordinate registration is concerned. Furthermore, electrical tilting is also much faster than its mechanical counterpart. However, even though electrical tilting is in principal faster than its mechanical counterpart, additional alignment procedures are usually required when the beam is shifted electrically from angle of incidence to another angle of incidence. These additional alignment also require a considerable amount time. Therefore, stereoscopic visualization is not routinely done in the semiconductor industry.
Accordingly, there is a need for a charged particle beam device which provides a sufficient resolution and which is able to increase the data collection to such an extent that the device can also be applied to high speed wafer inspection. Furthermore, there is a need for a charged particle beam device which is able to reduce the time that is needed to produce a pair of stereo images.